1. Field of the invention:
The present invention relates to a current-limiting fuse comprising an electrically conducting fusible element closely surrounded by a solid envelope made of non porous rigid material, in particular of high density ceramic. The invention also relates to a method of manufacturing such a fuse.
2. Brief description of the prior art:
Generally speaking, a fuse is an electric device designed to conduct a current and to interrupt this current when it reaches a predetermined value, in order to protect an electric circuit against a too high current. The very high fault currents are therefore interrupted well before their maximum amplitude is reached. Consequently, a fuse limits the energy developed in a faulty electric circuit so as to prevent damages thereto.
The conventional high power current-limiting fuses usually comprise an electrically insulating tube made of fiberglass or of ceramic and closed at each end by metallic closures. Such closures constitute terminals for the connection of the fuse in an electric circuit to be protected. Such conventional fuses also enclose at least one electrically conducting fusible element in the form of a wire or ribbon and having its two ends respectively connected to the two metallic closures. The fusible elements are made of metals such as silver, copper, aluminum, and so on, and are surrounded b an arc constricting agent, usually consisting of packed quartz sand which fills the insulating tube.
When a fault current flows through the fusible element, the metal of the same heats and reaches its melting point at locations determined by its geometry. A current interrupting electric arc is then produced, whose resistance increases up to a value sufficient to develop an arc voltage higher than the voltage of the source. As this arc voltage has a polarity opposite to that of the source voltage, it forces the fault current to a zero value. The characteristics of the fault current decrease are closely related to the nature of the arc constricting agent.
As the quartz sand has a low thermal conductivity and only partly fills (about 70%) the inner volume of the insulating tube, a low dissipation of the heat produced by the electric arc results, and accordingly the time required by the fuse to interrupt the current and the energy developed in the fuse both increase. Upon arcing, the metal of the fusible element is vaporized and an internal pressure is created. The so created pressure displaces the particles of the quartz sand to form a void having dimensions greater than the initial ones of the fusible element. A slow rise in arc voltage then results, while the time required for interrupting the current increases.
In order to increase the thermal conductivity and the mechanical rigidity of the quartz sand, U.S. Pat. Nos. 3,838,375 (FRIND et AL) issued on Sept. 24, 1974, and 4,003,129 (KOCH et AL) issued on Jan. 18, 1977, disclose binding of the quartz sand by means of an inorganic binder. The binder is so selected that the porosity of the arc constricting agent is not affected. Improved performance is obtained with fuses using as arc constricting agent bound quartz sand in comparison with the conventional fuses using classically packed sand.